A packet transmission system according to these features is the European WLAN standard High Performance Radio Local Area Network type 2 (HIPERLAN/2), which was developed by the European Telecommunication Standardization Institute (ETSI) broadband radio access network project (BRAN). ETSI created the BRAN project to develop standards and specification for broadband radio access networks that cover a wide range of applications and are intended for different frequency bands. The scope of the HIPERLAN/2 Technical Specifications are limited to the air interface, the service interfaces of the wireless subsystem, the convergence layer functions and supporting capabilities required to realize the services. Thus the HIPERLAN/2 technical specifications describe only the Physical (PHY) layer and the Data Link Control (DLC) layer, which are core network independent, and the core network specific convergence layer. Network layer and higher layer that are required for a complete system are not subject of the HIPERLAN/2 specification. These specifications are assumed to be available or to be developed by other bodies.
HIPERLAN/2 is designed as a high speed radio communication system with typical data rates from 6 Mbit/s to 54 Mbit/s. It connects portable devices with broadband networks that are based on Internet Protocol (IP), Asynchronous Transfer Mode (ATM) and other technologies. In addition to a centralized mode which is used to operate HIPERLAN/2 as an access network via a fixed access point also a capability for direct link communication is provided. HIPERLAN/2 systems are intended to be operated in the 5 GHz band with a power limit of 1 W mean EIRP (equivalent isotropic radiated power).
HIPERLAN/2 uses the multicarrier scheme Orthogonal Frequency Division Multiplexing (OFDM) which is known to be very robust in frequency selective environments. In the HIPERLAN/2 standard several different coherent modulation schemes like BPSK, QPSK, 16-QAM and optional 64-QAM are specified which are used for subcarrier modulation. For forward error control convolutional codes with rates 1/2, 9/16 and 3/4 are specified which are obtained by puncturing of a convolutional mother code of rate 1/2. The combination of a modulation scheme and a code rate is denoted as physical layer mode. The possible resulting physical layer modes of HIPERLAN/2 are listed in Table 1:
TABLE 1Physical layer modes of HIPERLAN/2Modulation schemecode ratebit rateBPSK½ 6 MbpsBPSK¾ 9 MbpsQPSK½12 MbpsQPSK¾18 Mbps16-QAM 9/1627 Mbps16-QAM¾36 Mbps64-QAM¾54 Mbps
In order to improve the radio link capability due to different interference situations and distance of mobile terminals (MTs) the appropriate physical layer mode is selected by a link adaptation scheme.
In HIPERLAN/2 data and control information are mapped onto transport channels. A packet is called Protocol Data Unit (PDU). Six different Protocol Data Unit types for different transport channels are specified                BCH-PDU PDU for Broadcast Channel        FCH-PDU PDU for Frame Channel        ACH-PDU PDU for Access Feedback Channel        SCH-PDU PDU for Short Transport Channel        LCH-PDU PDU for Long Transport Channel        RCH-PDU PDU for Random Channel        
Several PDUS are combined in the Data Link Control(DLC) layer to a PDU train. To establish different data links to different applications, so called Data Link Control Connections (DLC connections), a PDU train may consist of several sequences (in the following also called sub-trains; depicted in FIG. 4). E.g. one DLC connection is established for exchanging e-mail data between an e-mail client application and an e-mail server application and another data layer connection is established for exchanging data between an Internet browser and an Internet server.
The Data Link Control layer of HIPERLAN/2 specifies six different PDU train types:                Broadcast PDU train        FCH and ACH PDU train        Downlink PDU train        Uplink PDU train with short preamble        Uplink PDU train with long preamble        Direct link PDU train        
The BCH, FCH and ACH contain control information and they are mapped together to one PDU train. In case of multiple antennas the BCH builds the separate Broadcast PDU train and the FCH and ACH for each antenna together the FCH and ACH PDU train. The downlink and the uplink PDU trains contain PDUs of several DLC connections of the same mobile terminal (MT) each containing short transport channel (SCH) as well as long transport channel (LCH). The last four PDU train types (Downlink PDU train, Uplink PDU train with short preamble, Uplink PDU train with long preamble, Direct link PDU train)are processed by an encoder as a whole. Tail bits are appended to each train for code termination purposes. These bits, denoted as tail bits, return the convolutional encoder to a definite initial status, the “zero state”. Code termination also is performed for the BCH, FCH and ACH for each PDU separately. This also makes decoding to perform better as a PDU train will end up in a predefined state.
All SCHs of one Data Link Control connection are transmitted with the same physical layer mode. This is also valid for all LCHs of one Data Link Control (DLC) connection. The physical layer mode of SCHs and LCHs of one DLC connection may differ. Also different DLC connections within one PDU train are allowed to use different physical layer modes. As by this in the downlink and in the uplink PDU train a code termination is performed only once per PDU train problems may occur by changing the physical layer mode within a PDU train.
For example assuming the case where the SCHs of one DLC connection are transmitted with the very robust 6 Mbps mode and the following LCHs with the more sensitive physical layer 36 Mbps mode. Therefore the resulting error rates after the decoding of the different PDU types differ significantly. In that case the error rate of the SCHs increases because of the relatively high error rate of the following PDUS. This is because memory is inherent in the decoding process and no code termination is performed between the PDUs with different error protection properties.
There are several reasons why the physical layer mode of the SCHs and LCHs of one DLC connection has been allowed to differ. One reason is that SCHs carry more sensitive information like acknowledgements and therefore the error protection should be as high as possible. Therefore in HIPERLAN/2 only the 6, 9 and 18 Mbps modes have been allowed for the SCHs. The LCHs carry data information. If the physical layer mode is chosen too low there will be no errors but the throughput is restricted by the physical layer mode. If the physical layer mode is chosen to high the error rate is too high and extensive retransmissions will reduce the throughput drastically. The optimum of course depends on an actual transmission situation but to give a coarse orientation  a relatively high error rate of 10% may be the optimum for a maximal throughput.
In the following to compare the error protection properties between two packets, packets are denoted to have a higher error protection level, when the packet shows more robustness for transmission errors etc. than the packet it is compared to. Vice versa a packet is denoted to have a lower error protection level if it is more sensitive for transmission errors than another packet it is compared to.
An exemplary result of Bit Error Rate (BER) over time for two packet types with different error protection level is shown in FIG. 5a. A first packet PDUN is transmitted at a low error protection level, e.g. encoded with a coding rate of 3/4 and modulated by QPSK.
Therefore the resulting Bit Error Rate BER is relatively high. A second packet PDUN+1 following this first packet PDUN is transmitted at a higher error protection level, e.g. modulated as BPSK and encoded by a code rate of 1/2. Due to the high error protection level of the second packet PDUN+1 the Bit Error Rate falls to a lower level. In case the first packet PDUN is provided with a code termination the Bit Error Rate will fall immediately in one step to that lower level at the border between the first packet PDUN and the second packet PDUN+1 (indicated by the dashed line in FIG. 5a).
The solid curve shows the degradation of the error rate at the border from the first packet PDUN to the second packet PDUN+1 if no code termination is applied to the first packet PDUN. In both cases when decoding the second packet PDUN+1 the decoder will use samples from the end of the first packet PDUN. In case of code termination these samples will have already converged the decoder into a certain internal state so that the decoding process of the second packet PDUN+1 will continue with very reliable samples. In case of a missing code termination the decoder has to use samples from the end of the first packet PDUN that are less reliable. Thus the high Bit Error Rate of these samples extends behind the end of the first packet PDUN into the beginning section of the second packet PDUN+1. With ongoing decoding less samples of the end of the first packet are used and the impact of the low error protection level of the first packet PDUN onto the second packet PDUN+1 decreases. When no more samples of the first packet PDUN are used for decoding the Bit Error Rate has reached the typical level of the Bit Error Rate for the physical layer mode of the second packet PDUN+1.
The same effect, i.e. a degradation of the error rate occurs if low error protected packets proceed on highly error protected packets. In this case the Bit Error Rate starts to increase already in the midst of the first packet (FIG. 6a).
It is a shortcoming of the state of the art that in a packet transmission system the performance is affected when the error protection level of succeeding packets is changed within a packet transmission sequence without providing code termination. The straight forward idea to introduce an additional code termination by inserting an appropriate number of appropriate tail bits between those packets will mean that the system specification has to be changed accordingly.